46th International Workshop on High Energy Density Physics with Intense Ion and Laser Beams

Understanding radiative energy transport in stellar interiors requires accurate knowledge of the opacity of dense hydrogen plasmas, yet direct experimental constraints in the relevant regime have been absent. We report measurements of hydrogen opacity at extreme densities—up to ~800× solid density—and temperatures of a few million kelvin, achieved through a tailored low-velocity capsule implosion at the National Ignition Facility. This approach produces conditions comparable to those in red dwarf stellar cores while suppressing background emission sufficiently to enable time-resolved X-ray radiography. By tracking X-ray transmission during stagnation, we extract the density evolution and opacity of the transient plasma. The measured opacities show significant deviations from commonly used stellar opacity models (e.g., OPLIB) and instead align with modern atomistic simulations such as average atom models. These results provide the first experimental benchmark of hydrogen opacity in the dense-plasma regime that dominates low-mass stellar interiors. Beyond astrophysics, the implosion design and diagnostics developed here are applicable to advanced concepts for inertial fusion energy, where accurate radiation transport models are likewise essential.